Conventional processes for preparing CHDM generally comprise ring hydrogenation of aromatic dicarboxylic acid dialkyl ester to give cyclohexanedicarboxylic acid dialkyl ester (first reaction) and hydrogenation of its ester groups to give CHDM (second reaction).
Among these catalysts used for the respective reactions, known as effective for the first reaction are palladium, nickel, ruthenium, rhodium and the like (U.S. Pat. No. 3,334,149, Japanese Unexamined Patent Publications Nos. 163554/1979 and 192146/1994, U.S. Pat. No. 5,286,898 and U.S. Pat. No. 5,399,742); and known as effective for the second reaction are copper-chromite, copper oxide/zinc oxide, copper oxide/titanium oxide, copper oxide/iron oxide and catalysts prepared by modifying these copper-based catalysts with oxides of barium, magnesium and zinc and reducing the modified catalysts for activation (U.S. Pat. No. 3,334,149, Japanese Unexamined Patent Publications Nos. 192146/1994 and 196549/1995, U.S. Pat. No. 5,334,779 and U.S. Pat. No. 5,030,771, U.S. Pat. No. 4,929,777).
As the mode of reaction, a fixed-bed continuous reaction process is considered to be advantageous over a suspended catalyst reaction process in terms of productivity and yield. Herein, the fixed-bed continuous reaction process includes a reaction process (downflow-type process) wherein a preformed catalyst is placed into a pressure-resistant reactor, into which hydrogen and a raw material are supplied to the top of the reactor at a predetermined temperature and hydrogen pressure, and the reaction product is withdrawn from the bottom of the reactor; and a reaction process (upflow-type process) wherein hydrogen and a raw material are supplied to the bottom of the reactor and the reaction product is removed from the top of the reactor. The suspended catalyst reaction process comprises suspending a catalyst powder in an aromatic dicarboxylic acid diester or a cyclohexanedicarboxylic acid diester and subjecting the suspension to a reaction with heating while being pressurized with hydrogen.
As the examples of fixed-bed continuous reaction process, reported are processes comprising ring hydrogenation of a terephthalic acid dialkyl ester in the presence of a preformed supported ruthenium catalyst to give 1,4-cyclohexanedicarboxylic acid dialkyl ester in the first reaction (Japanese Unexamined Patent Publications Nos. 163554/1979 and 192146/1994).
Ruthenium catalysts are inexpensive compared with palladium catalysts, and exhibit a high activity even at a low pressure and a low temperature, but have the disadvantage of being likely to cause undesirable reactions involving high exothermic heat, such as hydrogenolysis of ester groups to hydroxymethyl group(s) or methyl group(s), in addition to the hydrogenation of the aromatic ring.
It is advantageous to use a cyclohexanedicarboxylic acid diester as a reaction solvent in order to avoid adverse effect due to reaction heat. However, the cyclohexanedicarboxylic acid diester used may be consumed due to the above-mentioned side reactions, not only resulting in a low yield but also, in extreme case, leading to a rapid generation of heat in a portion of the reactor which makes it difficult to continue the reaction. Therefore, Japanese Unexamined Patent Publication No. 192146/1994 proposes the provision of a perforated plate in the reactor to improve the dispersibility of gas and liquid. However, even in this case, the concentration of terephthalic acid dialkyl ester in the feed to the reactor actually must be limited to an extremely low range of 5 to 20 weight %, and a large amount of reaction product is subjected to reaction by recycling, resulting in a low yield based on the terephthalic acid dialkyl ester used and leading to a low productivity.
Examples of the fixed-bed continuous reaction process for the second reaction are those disclosed in Japanese Unexamined Patent Publications. Nos. 196549/1995; 196560/1995; 196558/1995; 196559/1995; 188077/1995; 188078/1995 and 188079/1995. These proposed processes are characterized in that the reaction is conducted under gas phase conditions of relatively low hydrogen pressure. However, the processes entail various disadvantages such as loss of thermal energy in vaporizing the raw material and necessity for removal of generated reaction heat in a gas phase of low heat conductivity, resulting in a need for complicated equipments. Furthermore, high-boiling-point by-products are deposited on the surface of catalyst, thereby markedly reducing the catalyst activity and thus necessitating frequent catalyst replacement or catalyst regeneration treatment.
On the other hand, U.S. Pat. Nos. 3,334,149, 5,030,771 and 5,334,779 and Japanese Unexamined Patent Publication No. 192146/1994 disclose a gas-liquid mixed phase reaction. However, the disclosed processes include various problems. For example, the feed rate (F/V) of cyclohexanedicarboxylic acid dialkyl ester is as low as 1/h or less, leading to a low productivity per reactor. Alternatively, cyclohexanedicarboxylic acid dialkyl ester is fed as diluted with the reaction product, i.e. CHDM or the like to a concentration of about 16% by weight or less, consequently involving complicated equipment and cumbersome operation relating to the reactor and resulting in increased by-products due to side reaction of CHDM.